A Strong X-ray Flare from a Likely z>1 AGN Adjoining the JWST NEP-TDF
ATel #11906; W. P. Maksym, F. Civano, C. MacLeod (CfA), R. Jansen, R. Windhorst, T. Ashcraft, V. Jones, S. Cohen (ASU), A. Koekemoer, N. Grogin (STScI), N. Cappelluti (Miami), C. Willmer (Arizona), M. Elvis, G. Fazio, M. Ashby (CfA), G. Hasinger (ESA), B. Cotton, J. Condon, W. Brisken, R. Perley (NRAO)
on 3 Aug 2018; 01:52 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Peter Maksym (peter.maksym@gmail.com)
Subjects: Radio, Infra-Red, Optical, Ultra-Violet, X-ray, Request for Observations, AGN, Quasar, Transient
Referred to by ATel #: 12049
We report discovery of an X-ray transient in recent (2018 July 26, 31) Chandra observations targeting the James Webb Space Telescope North Ecliptic Pole Time Domain Field (JWST NEP-TDF).
JWST NEP-TDF is a JWST Guaranteed Time Observer program (PI: Windhorst) with multi-epoch visits planned promptly after JWST launch (Jansen & Windhorst, arXiv:1807.05278). NEP-TDF has exceptional advantages for time domain science and early universe science with JWST due to its low IR background, lack of IR-bright interlopers, and location in the JWST continuous viewing zone. We are in the process of developing a deep multi-wavelength dataset to complement future observations and set a baseline for post-launch time domain science.
To date, three ~50-ks epochs of the associated 900-ks multi-cycle Chandra campaign have been observed. The source was marginally detected (~13 net 0.3-8 keV counts; SNR~2.3) over 2018 April 11-13 (obsids 21072, 21073, 20363). On 2018 July 26, we detected the source at SNR~9.5, with an increase in count rate by a factor of ~7 (neglecting modest corrections due to changes in vignetting and PSF). By 2018 July 31, the count rate had decayed by ~25% (variability SNR~2.3). Spectral fits of both July detections are consistent with an absorbed power law (nH~2e22/cm^2, Gamma~2, ~4e-14 cgs flux (0.3-8 keV)).
The location (RA, Dec)=(17:24:21.72, +65:48:47.3) is consistent with SDSS J172421.74+654847.5 (ugriz=[20.89,20.73,20.45,20.28,20.58]). Pre-flare GALEX and SDSS photometry (Richards et al, 2009, ApJS, 180, 67) indicate that the SDSS counterpart is likely a quasar at z~1.1 (1.0-1.6) but UV/O/IR host spectroscopy is currently unavailable. This interpretation is consistent with WISE W1,2,3 colors. For a flare at z~1.1, the X-ray luminosity is Lx~3e44 erg/s and we infer Lbol>10^45 erg/s based on pre-flare photometry. A redshift of z~1.44 is permitted and would place the source at a mere ~3.5 Mpc from NLSy1/FSRQ VCS5 J172314.1+654746, near the center of the field. Subaru HEROES giz photometry (PI: Hasinger) from 2017 July indicates brightening by (0.54,0.23,0.67) over SDSS magnitudes. No radio source is evident from our deep (rms~micro-Jy) pre-flare 3GHz VLA data.
This flare is strong for an AGN, and could indicate an accretion state change (e.g. a changing look quasar). A z>1 changing look quasar would be exceptional for its distance, would constrain the X-ray flare onset to <50 days (rest-frame), and would permit monitoring of several rest-frame UV transitions from the ground. At 10' from the NEP-TDF center, it is outside the planned JWST GTO program (r~NIRCAM<7', r~NIRISS<5') but would be well-situated for JWST GO observations with pure parallels that deepen or expand the GTO field.
We are planning multi-wavelength follow-up observations and encourage additional follow-up. O/IR spectroscopy is particularly needed to confirm the redshift and nature of the optical counterpart, and in anticipation of continued monitoring. Given the pre-flare SDSS magnitudes, such follow-up should be possible from a wide variety of telescopes.